Researchers at the Chinese Academy of Sciences Institute of Neuroscience in Shanghai have produced identical primate clones using the same procedure that brought Dolly the sheep into the world more than two decades earlier.

The procedure, known as somatic cell nuclear transfer, involves removing the nucleus of an egg cell and replacing it with nuclear material from a body cell.

Two genetically identical long-tailed macaque monkeys, named Zhong Zhong and Hua Hua, were produced from fibroblast cells taken from a monkey fetus. (The names come from the Chinese adjective “Zhonghua,” which applies to the Chinese people or nation.)

The world’s first primate clone was a rhesus monkey produced in 1999 using a simpler method known as embryo splitting, which parallels the natural process that results in identical twins. The Chinese experiment was the first to use a more powerful method called somatic cell nuclear transfer, or SCNT.

SCNT could open the way for creating thousands of genetically identical monkeys, and for conducting new types of genetic experiments.

“You can produce cloned monkeys with the same genetic background except the gene you manipulated,” senior study author Qiang Sun, director of the institute’s Nonhuman Primate Research Facility, said in a news release. “This will generate real models not just for genetically based brain diseases, but also cancer, immune, or metabolic disorders and allow us to test the efficacy of the drugs for these conditions before clinical use.”

Scientists have tried to extend the SCNT cloning technique to primates for years, but the procedure is fraught with difficulty. To overcome the hurdles, the Chinese research team added chemicals after the nuclear transfer to flip the molecular switches that regulate embryo development.

The feat still wasn’t easy.

The team went through more than 400 egg cells in all, and transferred 260 embryos into monkey surrogates. Two live births were reported when adult donor cells was used, but those two clones didn’t live for more than a few hours. Zhong Zhong and Hua Hua — both derived from fetal fibroblasts, a type of cell that matures into connective tissue — were the only baby monkeys to survive.

“We tried several different methods, but only one worked,” Sun said. “There was much failure before we found a way to successfully clone a monkey.”

Zhong Zhong and Hua Hua are currently being bottle-fed and are growing normally. The Chinese researchers are expecting more monkey clones to be born in the months ahead.

The Chinese lab is following international guidelines for animal research, established in cooperation with the U.S. National Institutes of Health. But the study authors say that the scientific community should have further discussions about what’s acceptable when it comes to cloning non-human primates.

“We are very aware that future research using non-human primates anywhere in the world depends on scientists following very strict ethical standards,” said co-author Mu-ming Poo, who helped supervise the experiment.

During a teleconference with reporters, Poo acknowledged that the technical barrier to “the cloning of primate species, including humans … is now broken.” However, he stressed that “there is no intention to apply this method to humans.”

It remains a very inefficient and hazardous procedure.

Experts have strongly opposed human cloning, citing ethical concerns about issues such as the low success rate and the risks of the procedure.

“It remains a very inefficient and hazardous procedure,” Robin Lovell-Badge, a cloning expert at the Francis Crick Institute in London, told Reuters. “The work in this paper is not a steppingstone to establishing methods for obtaining live born human clones. This clearly remains a very foolish thing to attempt.”

Animal welfare groups such as the People for the Ethical Treatment of Animals voiced opposition to further use of the procedure on monkeys as well. In an online statement, PETA UK described cloning as a “Frankenscience horror show.”